Electrochemical Method for the Production of Graphene Composites and Cell for Conducting the Same

ABSTRACT

A method of making an electrically conductive composite includes applying graphene oxide ( 27 ) to at least one non-conductive porous substrate ( 25 ) and then reducing the graphene oxide ( 27 ) to graphene via an electrochemical reaction. An electrochemical cell ( 10 ) for causing a reaction that produces an electrically conductive composite includes a first electrode ( 13 ), a second electrode ( 15 ), an ion conductive medium ( 17 ), electrical current in communication with the first electrode, and an optional third electrode having a known electrode potential. The first electrode ( 13 ) contains at least one layered electrocatalyst, which includes at least one non-conductive porous substrate ( 25 ) coated with graphene oxide ( 27 ) and at least a first and second active metal layer ( 29   a,    29   b ) comprising a conductive metal in contact with the non-conductive porous substrate ( 25 ) coated with graphene oxide ( 27 ).

FIELD OF THE INVENTION

The present invention is directed to electrochemical methods, and moreparticularly, to electrochemical methods for the production of graphenecomposites and electrochemical cells for conducting the same.

BACKGROUND OF THE INVENTION

Graphene is a 2D monolayer of sp²-hybridized carbon atoms arranged in ahoneycomb network. This material is known to have excellent physical andchemical properties for a wide variety of applications.

Graphene is synthesized through several different approaches. Forinstance, micromechanical cleavage techniques, liquid exfoliationtechniques, epitaxial growth techniques, total organic synthesistechniques, and chemical vapor deposition techniques have all been usedto varying degrees of success to form graphene.

An alternative route to bulk synthesis of graphene sheets is reductionof graphene oxide. This reduction may take place by way of a chemicalreduction, a thermal reduction, or an electrochemical reduction, wherethe electrochemical reduction is used to produce a catalyst with the aidof a co-deposited metal and the formation of a metal nanostructure. Whencompared to chemical reduction, electrochemical reduction of grapheneoxide is both less onerous and less detrimental to the environment. Theelectrochemical approach changes the electronic states of graphene oxidethrough the application of an external power supply, which eliminatesthe need for hazardous chemical reducing agents, such as hydrazine andits derivatives. The electrochemical approach also avoids contaminationfrom excess reducing agents. However, it has only been used for theco-deposition of metals and the synthesis of nanostructure metalcomposites.

Electrochemical coating of graphene-metal composites on conductivesubstrates, such as glassy carbon, gold disks, or metal foils, has beenreported and has found some utility for the fabrication of sensors,hydrogen production, and energy storage. However, incorporation ofgraphene into isolating substrates, i.e., non-conducting poroussubstrates, is much more difficult. Due to the intrinsic isolatingproperties of the component materials, e.g., polymer, glass, and wood,these porous substrates cannot be used in applications that requireconductive materials, such as electrochemical electrodes, electronics,sensors, and switches.

Thus, a need exists for methods for creating composites of graphene onnon-conductive porous substrates.

SUMMARY OF THE INVENTION

The present invention is premised on the realization thatelectrochemical methods could be used to form graphene composites onnon-conductive porous substrates. More particularly, the presentinvention is premised on the realization that non-conductive poroussubstrates coated with graphene oxide could undergo electrochemicalreduction to form such composites. Incorporating graphene intonon-conducting porous substrates, such as paper, clothing fabrics, andfibers, could allow for an expansion of the available materials fordevelopment of novel electronic systems and components. Suchnon-conductive porous substrates have been widely used in the phaseand/or size separation of certain molecules, the immobilization ofenzymes or microorganisms in biotechnology, as well as in otherapplications.

In accordance with an embodiment of the present invention, a method ofmaking an electrically conductive composite is provided. The methodincludes applying graphene oxide to at least one non-conductive poroussubstrate and then reducing the graphene oxide to graphene via anelectrochemical reaction.

In accordance with another embodiment of the present invention, anelectrochemical cell for causing a reaction that produces anelectrically conductive composite is provided. The electrochemical cellincludes a first electrode containing at least one layeredelectrocatalyst. This electrocatalyst includes at least onenon-conductive porous substrate coated with graphene oxide and at leasta first and second active metal layer comprising a conductive metal incontact with the non-conductive porous substrate coated with grapheneoxide. The electrochemical cell further includes a second electrode, anion conductive medium, electrical current in communication with thefirst electrode, and an optional third electrode having a knownelectrode potential.

The objects and advantages of the present invention will be furtherappreciated in light of the following detailed description and examplesin which:

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with the general description of the invention given above, andthe detailed description given below, serve to describe the invention.

FIG. 1 is a diagrammatical view of a simplified electrochemical cell, inaccordance with an embodiment of the present invention.

FIG. 1A is an enlarged view of the encircled portion 1A of the firstelectrode shown in FIG. 1, showing in cross-section the non-conductiveporous substrate, graphene oxide coating material, and active metallayers, in accordance with an embodiment of the present invention.

FIG. 2 shows photographs of the graphene oxide coated filter paperexample, in accordance with an embodiment of the present invention.

FIG. 3 shows photographs of the graphene coated filter paper, inaccordance with an embodiment of the present invention.

FIG. 4 shows the X-Ray diffraction data from a filter paper, a grapheneoxide coated filter paper, and a graphene coated filter paper, inaccordance with an embodiment of the present invention.

FIG. 5 shows the Raman spectroscopy data from a filter paper, a grapheneoxide coated filter paper, and a graphene coated filter paper, inaccordance with an embodiment of the present invention.

FIG. 6 shows a photograph of the results obtained from a qualitativehydrophobicity experiment performed using filter paper and a graphenecoated filter paper, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Unless clearly defined otherwise from the context, any range of valuespresented in the following Detailed Description and Claims includes eachend point as well as each whole number or fractional part thereof,within the recited range. Additionally, approximating language may beapplied to modify any quantitative representation that may vary withoutresulting in a change in the basic function to which it is related.Accordingly, a value modified by a term or terms, such as “about” and“substantially,” may not be limited to the precise value specified. Byway of example, a range from “about 0.8 nm to about 50 nm,” includes forexample, 0.8 nm, 0.9 nm, 1 nm, 3 nm, 3.14 nm, 49.999 nm, 50 nm, etc.,and may include values slightly below 0.8 nm and slightly above 50 nm.

In accordance with an embodiment of the present invention and inreference to FIG. 1, an electrochemical cell 10 is provided thatincludes a first electrode 13, a second electrode 15, an ion conductivemedium 17 in electrolytic communication with the first and secondelectrodes 13, 15, and a voltage source 19 in electrolytic communicationwith the first and second electrodes. The first and second electrodes13, 15 may be physically separated by a separator 18. In a batch-typearrangement shown in FIG. 1, the ion conductive medium 17 may be stirredto facilitate mass transfer. It will be readily apparent to one ofordinary skill in the art that the above cell 10 is readily adaptable toa continuous flow cell configuration, semi-continuous, and withrecirculation of the ion conductive medium 17.

In accordance with embodiments of the present invention and as shown inFIG. 1A, the first electrode 13 includes a non-conductive poroussubstrate 25 coated with graphene oxide coating 27 at least partiallycovering the non-conductive porous substrate 25 and also at leastpartially coating the inside network of the non-conductive poroussubstrate 25. Additionally, the first electrode 13 includes at least afirst and second active metal layer 29 a, 29 b in contact with thenon-conductive porous substrate 25 coated with a graphene oxide coating27.

For purposes of the present invention, the non-conductive poroussubstrate is not particularly limited and includes anything that is notconductive, or only sparingly so, and porous, i.e., having minute spacesinto which liquids and/or gases may infiltrate. Non-conductive poroussubstrates include fabric, articles of clothing, paper, polymermembranes and films, glass materials, wood, cotton, certain fibrousmaterials, and many other materials.

The materials used for active metal layers 29 a, 29 b are notparticularly limited and may be utilized in a wide range of applicationsand processes. For example, the materials include, but are not limitedto, platinum, iridium, rhodium, rubidium, ruthenium, rhenium, palladium,gold, silver, nickel, iron, cobalt, copper, zinc, chromium, tantalum,gallium, cadmium, indium, thallium, tin, lead, bismuth, silver, mercury,niobium, vanadium, manganese, aluminum, arsenic, selenium, antimony,titanium, tungsten, Raney metal, carbon steel, stainless steel,graphite, and mixtures and alloys thereof. In one embodiment, the activemetal layers 29 a, 29 b comprise nickel and/or nickel oxyhydroxide. Inanother embodiment, the active metal layers 29 a, 29 b comprise atitanium in the form of a foil or a mesh. In another embodiment, theactive metal layers 29 a, 29 b comprise nickel, manganese, cobalt, orzinc, or combinations thereof.

In accordance with an embodiment, the graphene oxide coating 27 may beone graphene oxide layer thick. In another embodiment, the grapheneoxide coating 27 may be comprised of multiple graphene oxide layers. Forexample, the graphene oxide coating 27 may comprise 1, 2, 3, 4, or 5 ormore, graphene oxide layers. Accordingly, the graphene oxide coating 27may be comprised of 1 to about 5 layers of graphene oxide.Alternatively, the graphene oxide coating may be comprised of about 3 toabout 5 layers of graphene oxide. Thus, “few layer graphene oxidesheets” as used herein may be comprised of about 2 to about 5 layers ofgraphene oxide. Accordingly, the graphene oxide coating may have athickness from about 0.8 nm to about 50 nm, or about 0.9 nm to about 25nm, or about 1 nm to about 10 nm, or about 2 nm to about 5 nm, forexample.

The second electrode 15 comprising a conductor is not particularlylimited and may be selected from electrodes known in the art. Forinstance, the second electrode 15 may comprise, but is not limited to,platinum, palladium, carbon, rhenium, nickel, Raney Nickel, iridium,vanadium, cobalt, iron, ruthenium, molybdenum, carbon steel, titanium,or combinations thereof. In some embodiments, the second electrodeserves to oxidize water, and so any electrode capable of carrying outsuch an oxidation is appropriate for use as the second electrode. It iswithin the purview of one of ordinary skill in the art to select theappropriate second electrode in view of the parameters of the particularapplication for which the inventive system is used.

Although not shown, the electrochemical cell of the present inventionmay include a third electrode. This third electrode may be a referenceelectrode to monitor the voltage applied to the electrochemical cell,such as an Ag/AgCl electrode, an Hg/HgO electrode, a calomel electrode,or a hydrogen reversible electrode. Many reference electrodes are knownand it is within the purview of one of ordinary skill in the art toselect the appropriate reference electrode in view of the parameters ofthe particular application for which the inventive system is used.

The ion conductive medium 17 can be aqueous or non aqueous and acidic,basic, or pH-neutral. In an embodiment, the ion conductive medium 17comprises organic solvents or ionic liquids. In another embodiment, theion conductive medium 17 is an aqueous, basic electrolyte solution. Forexample, an aqueous, basic electrolyte solution may comprise a hydroxidesalt (e.g., KOH, NaOH), a carbonate salt (e.g., K₂CO₃, Na₂CO₃), abicarbonate salt (e.g., KHCO₃, NaHCO₃), or combinations thereof.Alternatively, the ion conductive medium 17 may be an aqueous solutionof sodium chloride.

The ion conductive medium 17 may have a hydroxide concentration of lessthan 0.1 M. For example, the pH of the ion conductive medium 17 may bein a range from about 10 to about 7. Alternatively, the ion conductivemedium 17 may have a hydroxide concentration of about 0.1 M to about 5M. In another embodiment, the ion conductive medium 17 is an aqueous,pH-neutral electrolyte solution, such as an aqueous solution of sodiumchloride. In yet another embodiment, the electrolyte ion conductivemedium 17 is an aqueous, acidic electrolyte solution.

In accordance with another embodiment, the ion conductive medium 17includes a gel, such as a solid polymer electrolyte. Suitable gelsinclude, but are not limited to, those containing polyacrylic acid,polyacrylates, polymethacrylates, polyacrylamides, and similar polymersand copolymers.

The electrolytic gel may be prepared using any suitable method. Onemethod includes forming a polymer and then injecting a hydroxide, acarbonate or a bicarbonate salt electrolyte into the polymer to form apolymeric mixture. In another method, the monomer may be polymerized inthe presence of a hydroxide, a carbonate, or bicarbonate saltelectrolyte. It is within the purview of one of ordinary skill in theart to select the appropriate ion conductive medium in view of theparameters of the particular application for which the inventive systemis used.

Optional separator 18 compartmentalizes the first and second electrodes13, 15. Separators should be constructed from materials chemicallyresistant to the ion conductive medium 17. Many polymers are suitablefor constructing separators, such as Teflon® and polypropylene.Separators are not required for simple batch-type arrangements, but maybe advantageous for continuous flow electrochemical cells. Separatorsmay include ion exchange membranes, solid electrolytes, or the aboveelectrolytic gels, for example. Separators may be permeable,semi-permeable or impermeable to gases or liquids.

Voltage source 19 may be any available source, such as batteries, fuelcells, power from the grid, and renewable energy sources, such as asolar cell or a wind-turbine generator, for example. The voltage source19 is in electrolytic communication with the first and second electrodes13, 15, and provides a voltage difference sufficient to reduce thegraphene oxide to graphene.

In accordance with another embodiment of the present invention, a methodof making an electrically conductive composite is provided. The methodcomprises applying graphene oxide to at least one non-conductive poroussubstrate. Graphene oxide is then reduced to graphene via anelectrochemical reaction. This method may be performed in theelectrochemical cell described above, for example.

The graphene oxide may be applied to the non-conductive porous substrateby a variety of means. For instance, the graphene oxide may be appliedby spraying, ultrasonic spraying, dip coating, spinning, printing,soaking, rolling, or any other acceptable means. It is within thepurview of one of ordinary skill in the art to select the appropriatemethod of applying the graphene oxide in view of the parameters of theparticular application for which the inventive system is used.

Graphene oxide may be applied to the non-conductive porous substratewith a loading sufficient to provide an increase of the electronicconductivity of the material. This loading is expected to vary with theporosity and thickness of the substrate. Exemplary loadings of grapheneoxide in 0.1 mg/cm² and 1 mg/cm². It is within the purview of one ofordinary skill in the art to select the appropriate loading of thegraphene oxide in view of the parameters of the particular applicationfor which the inventive system is used.

During the electrochemical reaction, a potential is applied to theelectrochemical cell. The applied potential may range from about −0.1 Vto about −5 V or from about −0.5 V to about −4 V or from about −1 V toabout −3 V or from about −1.1 V to about −1.5 V. the applied potentialmay be about −1.2 V, for example. This potential may be applied forabout 1 second to about 120 minutes or from about 1 minute to about 60minutes. The potential may be applied for 35 minutes or 55 minutes, forexample. The actual amount of time required depends on the loading,porosity, and size of the electrochemical cell. For instance, if thesample is very small, the time required may be on the order of seconds,but if the sample is very large, the time required may be on the orderof hours. It is within the purview of one of ordinary skill in the artto select the appropriate potential to apply and the time of theapplication of that potential to effect the desired level of reductionof the graphene oxide.

The present invention will be further appreciated in view of thefollowing examples.

EXAMPLES

Filter paper was cut into 1.5 cm×1.5 cm pieces, rinsed with deionizedwater (DI water), then dried at 70° C. overnight in a model 30 GC laboven (Quincy Lab Inc.). Commercial graphene oxide suspension waspurchased from Angstron Materials (Batch # S2110514). The commercialgraphene oxide suspension was diluted to 0.1 mg/ml with DI water andsonicated for 10 minutes (Branson 2800 Ultrasonic Cleaner). Then, thediluted graphene oxide suspension was sprayed onto the previously cutfilter paper substrates using an airbrush (TCP Global) with Ar gas(Praxair) at about 70° C. on a hot plate (Corning PC-620D) to acceleratewater evaporation.

The electrochemical reduction was performed in a three-electrodeelectrochemical cell. The graphene oxide coated filter paper substrateswere sandwiched with two titanium foils and used as working electrodes.Plastic insulators, which are resistant to the electrolyte, are used tohold or press the titanium foils to the edges of the graphene oxidecoated filter paper. An Ag/AgCl electrode and platinum foil were used asthe reference electrode and counter electrode, respectively. A potentialof −1.2 V vs. Ag/AgCl was applied for the reduction of graphene oxide in1 M NaCl. The reduction times for the three electrodes of varyingloading are listed in Table 1. After electrochemical reduction, thegraphene coated filter paper substrates were rinsed with DI water anddried at 70° C., then weighed to obtain the graphene loading. This valueis reported as graphene loading in Table 1.

TABLE 1 Reduction Graphene Electrode Time (min) Loading (mg/cm²) a 350.05 b 35 0.18 c 55 0.58

The graphene oxide coated filter paper samples with varying loadings areshown in FIG. 2, while FIG. 3 shows the same samples afterelectrochemical reduction. As shown in the figures, the filter paperbecomes brown upon coating with graphene oxide and black after theelectrochemical reduction.

The samples were characterized by X-Ray diffraction analysis, as shownin FIG. 4, both after coating with graphene oxide and after theelectrochemical reduction. The peak at 11.3° in the curve obtained fromthe analysis of the graphene oxide coated filter paper is correlated tothe interlayer spacing between the graphene oxide nanosheets. Thus, thedisappearance of this peak at 11.3° in the curve obtained from theanalysis of the electrochemically reduced sample indicates the reductionof graphene oxide to graphene. Due to the huge diffraction peak of thefilter paper at about 20° to about 25°, the anticipated (002) reflectionof graphene at around 23° cannot be distinguished.

Similarly, the samples were characterized by Raman spectroscopy, asshown in FIG. 5. The Raman spectra of both the graphene oxide coatedfilter paper and the graphene composite on filter paper exhibit the Dand G bands at around 1350 cm⁻¹ and 1600 cm⁻¹, respectively. Theintensity ratio of the D band to the G band in the curve obtained fromthe graphene composite sample is increased relative to that of the curveobtained from the graphene oxide coated sample. The increased D/Gintensity ratio in the Raman spectra is consistent with significantstructural changes in the electrochemically reduced sample, suggestingthe reduction of graphene oxide.

The samples were further characterized as to their conductivity andresistance via a two probe multimeter with a 1 cm distance between thetwo probes. The filter paper and GO coated filter paper are isolating.However, after electrochemical reduction, the composites becomeconducting, as shown in Table 2. Table 2 further shows that theresistance of the composite decreases as the loading of grapheneincreases.

TABLE 2 Graphene Resistance before Resistance after Electrode Loading(mg/cm²) reduction (kΩ) reduction (kΩ) Control 0 >211,000 >211,000 a0.05 >211,000 6.6 b 0.18 >211,000 1.2 c 0.58 >211,000 0.78

Finally, the relative hydrophobicity of the uncoated filter paper and agraphene composite in accordance with the above discussion wasdetermined qualitatively by dropping a drop of water on each sample. Asshown in FIG. 6, the uncoated filter paper (I) absorbs water, while thewater drop is repelled by the graphene composite (II). Thus, thegraphene composite is hydrophobic.

While the present invention was illustrated by the description of one ormore embodiments thereof, and while the embodiments have been describedin considerable detail, they are not intended to restrict or in any waylimit the scope of the appended claims to such detail. Additionaladvantages and modifications will readily appear to those skilled in theart. The invention in its broader aspects is therefore not limited tothe specific details, representative product and method, andillustrative examples shown and described. Accordingly, departures maybe made from such details without departing from the scope of thegeneral inventive concept embraced by the following claims.

What is claimed is:
 1. A method of making an electrically conductivecomposite comprising: applying graphene oxide to at least onenon-conductive porous substrate; and reducing the graphene oxide tographene via an electrochemical reaction.
 2. The method of claim 1,wherein the applying graphene oxide is effected by at least one ofspraying, ultrasonic spraying, dip coating, spinning, printing, soaking,or rolling.
 3. The method of claim 1, wherein the at least onenon-conductive porous substrate is a fabric, an article of clothing, apaper, a polymer membrane, a polymer film, a glass, a wood, cotton, or afibrous material.
 4. The method of claim 1, wherein the electrochemicalreaction takes place in an electrochemical cell comprising: a firstelectrode comprising at least one layered electrocatalyst, wherein theat least one layered electrocatalyst comprises: the at least onenon-conductive porous substrate coated with graphene oxide; at least afirst and a second active metal layer comprising a conductive metal incontact with the at least one non-conductive porous substrate coatedwith graphene oxide; a second electrode comprising a conductor; an ionconductive medium; electrical current in communication with the firstelectrode; and optionally, a third electrode having a known electrodepotential.
 5. The method of claim 4, wherein the at least onenon-conductive porous substrate is a fabric, an article of clothing, apaper, a polymer membrane, a polymer film, a glass, a wood, or cotton.6. The method of claim 4, wherein the ion conductive medium has anacidic or a neutral pH.
 7. The method of claim 4, wherein the ionconductive medium has a basic pH.
 8. The method of claim 4, wherein theion conductive medium is an aqueous solution of sodium chloride.
 9. Themethod of claim 4, wherein the third electrode is an Ag/AgCl electrode,an Hg/HgO electrode, a calomel electrode, or a hydrogen reversibleelectrode.
 10. An electrochemical cell for causing a reaction thatproduces an electrically conductive composite, the electrochemical cellcomprising: a first electrode comprising at least one layeredelectrocatalyst, wherein the at least one layered electrocatalystcomprises: at least one non-conductive porous substrate coated withgraphene oxide; at least a first and a second active metal layercomprising a conductive metal in contact with the at least onenon-conductive porous substrate coated with graphene oxide; a secondelectrode comprising a conductor; an ion conductive medium; electricalcurrent in communication with the first electrode; and optionally, athird electrode having a known electrode potential.
 11. Theelectrochemical cell of claim 10, wherein the at least onenon-conductive porous substrate is a fabric, an article of clothing, apaper, a polymer membrane, a polymer film, a glass, a wood, cotton, or afibrous material.
 12. The electrochemical cell of claim 10, wherein theion conductive medium has an acidic or a neutral pH.
 13. Theelectrochemical cell of claim 10, wherein the ion conductive medium hasa basic pH.
 14. The electrochemical cell of claim 10, wherein the ionconductive medium is an aqueous solution of sodium chloride.
 15. Theelectrochemical cell of claim 10, wherein the third electrode is anAg/AgCl electrode.
 16. The electrochemical cell of claim 10, furthercomprising a separator between the first electrode and the secondelectrode.
 17. The electrochemical cell of claim 10, wherein the ionconductive medium comprises organic solvents or ionic liquids.
 18. Theelectrochemical cell of claim 10, wherein the ion conductive mediumcomprises an electrolytic gel.
 19. The electrochemical cell of claim 18,wherein the electrolytic gel comprises a polymer component selected fromthe group consisting of a polyacrylic acid, a polyacrylate, apolymethacrylate, a polyacrylamide, and a mixture thereof.
 20. Theelectrochemical cell of claim 10, wherein the graphene oxide has athickness of about 0.8 nm to about 50 nm.
 21. The electrochemical cellof claim 10, wherein a loading of the graphene oxide coated on the atleast one non-conductive porous substrate is between 0.1 mg/cm² and 1mg/cm².